Bus switch circuit with back-gate control during power down

ABSTRACT

The bus switch with back gate control circuit includes: an NMOS transistor coupled between a first port and a second port; a PMOS transistor coupled in parallel with the NMOS transistor; a first blocking device coupled between the first port and a control node of the PMOS transistor; a second blocking device coupled between the second port and the control node of the PMOS transistor; a first pull-down device coupled to a back gate of the NMOS transistor; and a second pull-down device coupled to the back gate of the NMOS transistor, wherein the pull down device is controlled by a power supply node and the control node of the PMOS transistor.

FIELD OF THE INVENTION

The present invention relates to electronic circuitry and, in particular, a bus switch circuit with back-gate control.

BACKGROUND OF THE INVENTION

On resistance (Ron) flatness is a concern in analog switches and one way to achieve a flatter Ron is by allowing the back-gate of the NMOS transistor to increase in voltage, producing a stronger transistor with less resistance. To allow the back-gate to rise an NMOS transistor is used as a switch to connect and disconnect the back-gate to its source. The drawback with this is during a power down condition the back-gate will be left floating and will allow higher leakage through the pass transistor.

SUMMARY OF THE INVENTION

The bus switch with back gate control circuit includes: an NMOS transistor coupled between a first port and a second port; a PMOS transistor coupled in parallel with the NMOS transistor; a first blocking device coupled between the first port and a control node of the PMOS transistor; a second blocking device coupled between the second port and the control node of the PMOS transistor; a first pull-down device coupled to a back gate of the NMOS transistor; and a second pull-down device coupled to the back gate of the NMOS transistor, wherein the pull down device is controlled by a power supply node and the control node of the PMOS transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

The Drawing is a circuit diagram of a preferred embodiment bus switch with back gate control, according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The power down circuit, according to the present invention, for bus switch with back-gate control, minimizes the leakage condition. The Drawing shows a preferred embodiment bus switch with back-gate control. The circuit shown in the Drawing includes PMOS transistors P1, P2, P3, P4, P5, and P6; NMOS transistors N1, N2, N3, N4, N5, NPD1, and NPD2; inverter INV1; diodes D1, D2, D3, and D4; Schottky diodes SD1 and SD2; control signals pmos_ctrl and nmos_ctrl; and ports A and B. Transistor P3 and diode SD1 form a blocking circuit. Transistor P4 and diode SD2 form a blocking circuit. Transistors NPD2, PS, and N4 form a pull down device for the back gate of transistor N1. This pull down device is controlled by the power supply and node s9. The preferred embodiment power down circuit provides an improvement over the prior art by accounting for the added leakage that is caused by the back-gate control circuit. Transistor NPD1 is the back-gate pull-down for normal operation. During a powered down condition node S2 will be low and transistor NPD1 will be off allowing node S1 to float. This is where the preferred embodiment circuit comes in. Transistor NPD2 is a pull down that is controlled by a CMOS device (transistors PS and N4). During normal operation, node S10 will be low and transistor NPD2 will be off, but during power down situations transistor PS turns on and allows node S10 to follow node S9, which can be controlled by external sources that maybe connected to port A or port B. Node S10 voltage will follow the higher of the two sources connected to port A and port B through transistor P3 and diode SD1, or transistor P4 and diode SD2, respectively. With either external source going high, transistor NPD2 will turn on, and node S1 will be held low and the potential between the back-gate of transistor N1 and its drain and source will be at its greatest. This condition will minimize the leakage through transistor N1.

The preferred embodiment device provides several advantages. It handles floating nodes that were created by the back-gate control in the prior art. And, it's small, simple, straight forward and uses very little area.

While this invention has been described with reference to an illustrative embodiment, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiment, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

1. A circuit comprising: an NMOS transistor coupled between a first port and a second port; a PMOS transistor coupled in parallel with the NMOS transistor; a first blocking circuit coupled between the first port and a control node of the PMOS transistor; a second blocking circuit coupled between the second port and the control node of the PMOS transistor; a first back gate pull-down transistor coupled to a back gate of the NMOS transistor; a second back gate pull-down transistor coupled to the back gate of the NMOS transistor; and a CMOS device coupled between the control node of the PMOS transistor and a ground node, and having an output coupled to a control node of the second pull-down transistor, and an input coupled to a supply node.
 2. The circuit of claim 1 wherein a control node of the first back gate pull-down transistor is inversely coupled to a control node of the NMOS transistor.
 3. The circuit of claim 1 further comprising: a first transistor coupled between the first port and the back gate of the NMOS transistor; and a second transistor coupled between the second port and the back gate of the NMOS transistor.
 4. The circuit of claim 3 wherein a control node of the first transistor is coupled to a control node. of the NMOS transistor, and a control node of the second transistor is coupled to the control node of the NMOS transistor.
 5. The circuit of claim 2 further comprising: a first transistor coupled between the first port and the back gate of the NMOS transistor; and a second transistor coupled between the second port and the back gate of the NMOS transistor.
 6. The circuit of claim 5 wherein a control node of the first transistor is coupled to the control node of the NMOS transistor, and a control node of the second transistor is coupled to the control node of the NMOS transistor.
 7. The circuit of claim 1 further comprising a back gate transistor coupled between the back gate of the PMOS transistor and the supply node.
 8. The circuit of claim 7 wherein a control node of the back gate transistor is coupled to a control node of the NMOS transistor.
 9. The circuit of claim 4 further comprising a back gate transistor coupled between the back gate of the PMOS transistor and the supply node.
 10. The circuit of claim 9 wherein a control node of the back gate transistor is coupled to a control node of the NMOS transistor.
 11. The circuit of claim 1 wherein the CMOS device comprises: a first transistor coupled between the output of the CMOS device and the ground node; and a second transistor coupled between the output of the CMOS device of the control node of the PMOS transistor.
 12. A circuit comprising: an NMOS transistor coupled between a first port and a second port; a PMOS transistor coupled in parallel with the NMOS transistor; a first blocking device coupled between the first port and a control node of the PMOS transistor; a second blocking device coupled between the second port and the control node of the PMOS transistor; a first pull-down device coupled to a back gate of the NMOS transistor; and a second pull-down device coupled to the back gate of the NMOS transistor, wherein the pull down device is controlled by a power supply node and the control node of the PMOS transistor.
 13. The circuit of claim 12 wherein the pull down device comprises: a pull down transistor coupled to the back gate of the NMOS transistor; and a CMOS device coupled between a ground node and the control node of the PMOS transistor, and having an output coupled to a control node of the pull down transistor, and having an input coupled to the power supply node. 